This invention relates to the manufacture of loose tube elements for use in fiber optic cables. More particularly, the invention relates to the manufacture of such elements with controlled fiber-to-sheath ratios over a wide range of production speeds.
As is well known, one method of assuring optimal transmission performance of optical fibers is to surround them in a grease-like material which is encased by a plastic sheath. This configuration is known as a "loose tube" construction. These loose tube elements can be subsequently stranded around a central strength member to form a cable, or alternatively can be individually overjacketed with a suitable strength member system to form a cable. The loose tube elements may contain individual optical fibers, or one or more bundles of optical fibers, or one or more ribbons containing arrays of parallel optical fibers bound together with a suitable binder, such as a UV curable acrylate composition. See for example, U.S. Pat. No. 4,826,278 to Garthside, III, et al.
Common to all of the above cable designs is that the optical fibers must be maintained in a stress-free condition in the finished cable. Moreover, this stress-free condition should be maintained across the operating temperature range of the cable once the cable is placed in its service location. A further constraint is that the fibers should not be subjected to excessive stress levels (i.e. approaching their "proof strength") either during the manufacturing process or during cable installation.
One means of assuring that the fibers are maintained in a stress-free condition in the final cable design is to control the ratio of the length of the fiber or fibers to the length of the surrounding protective strength. This fiber-to-sheath length ratio is well known in the industry as the "fiber excess length" or simply "XSL". The desired stress-free condition of the fibers can be maintained through control of the XSL, combined with the selection of suitable material and dimensions for the protective sheath and the exercise of prudence in the manufacturing process by placing minimum stress on the loose tube elements during the processing prior to coupling the elements to a strength member. A fiber-to-sheath length ratio of just over 1.000 is usually preferred, and a ratio well over 1.000 is detrimental.
One method of controlling the fiber-to-sheath length ratio is described in U.S. Pat. No. 4,414,165 to Oestreich, et al. According to that method a tubular jacket was formed around the fiber and grease-like material at an elevated temperature. The jacket was then simultaneously cooled and coiled with suitable back tension being applied to the fiber to align the fiber on the inner radius of curvature in the coiled configuration. Subsequently, the coiled configuration was cooled to ambient temperature and the shrinkage of the jacket, caused by cooling, produced the desired the fiber-to-sheath length ratio.
Another method of controlling the fiber-to-sheath length ratio is described in U.S. Pat. Nos. 4,772,435 and 4,893,998 to Schlaeppi, et al. According to that method, a withdrawal device pulled the sheath containing the fibers and grease-like material through a cooling vat while at the same time subjecting the fiber to a predetermined back tension. The sheath was subsequently pulled through a second cooling vat while subjecting the sheath to as little tensile stress as possible and keeping the fiber free of tensile stress. The sheath was cooled to a temperature in the second cooling vat which was lower than the temperature of the sheath in the first cooling vat.
The above methods all suffer in their ability to accommodate significant fluctuations in production line speed while at the same time maintaining a consistent fiber-to-sheath length ratio. Such fluctuations occur between start up speed and full line speed. Furthermore, these methods are limited in their maximum line speed above which the fiber-to-sheath length ratio begins to increase uncontrollably. This phenomenon can be explained as follows. If the distance between the extruder and the coiling device, which is pulling the sheath from the extruder, is fixed, and the cooling vat is placed in this fixed space, then as the line speed increases the temperature of the sheath as it engages the coiling device increases. If the fiber is to be aligned on the inner circumference of the sheath passing around the coiling device by applying a predetermined back tension to the fiber, then the temperature at which the fiber engages the inner circumference of the sheath increases with increasing line speed. Subsequent shrinkage of the sheath during cooling, which occurs when the sheath is coiled about the coiling device with the fiber engaging the inner circumference of the sheath, causes an increase in the fiber-to-sheath length ratio. Accordingly, the fiber-to-sheath length ratio increases with increasing line speed because the temperature of the sheath at the point where the fiber engages its inner circumference increases with increasing line speed. This causes an undesirably high fiber-to-sheath length ratio well over 1.000.
These deficiencies were noted in U.S. Pat. No. 4,814,116 to Oestreich, et al. That patent disclosed the use of a variable diameter haul off coiling device designed such that the diameter and corresponding effective length ratio between the fiber and the warm sheath could be varied as a function of line speed. Although that improvement benefitted the manufacture of a single-size tube over a limited range of line speeds, the ability of the haul off to provide a constant pulling speed on the extruded sheath was compromised because of flats which developed between the movable segments as the diameter of the haul off was expanded. The periodic variation in haul off speed manifested itself as sheath diameter fluctuations which compromised the quality of the finished product.
Other methods which have been used to attempt to control the fiber-to-sheath length ratio include (1) metering and controlling the length of fiber entering the extruder cross head as a function of the speed of the exit capstan; (2) heating the tube in an auxiliary furnace to promote residual shrink back of the plastic tubular sheath; (3) propelling the fiber into the tube via the grease velocity; (4) utilizing multiple storage disk cooling chambers; (5) elongating the plastic sheath just prior to winding on a storage reel via the use of a dual capstan with a fixed speed differential; and (6) twisting the composite loose tube and fibers during the manufacturing step.
None of the above-mentioned methods or apparatuses provide totally satisfactory solutions to the problem of controlling the fiber-to-sheath length ratio in a loose tube stranding element or central core element of a fiber optic cable. This is especially true when the production line is to be operated over a wide range of line speeds and with a variety sheath diameters required for different products. Prior art methods and apparatuses also have difficulty in achieving high line speeds with the recently developed grease-like materials due to their viscosity, higher critical yield stress and higher drip resistant ratings.